The present invention relates to a pulverized coal burner which is a type of pulverized coal float-firing burner and, more particularly, to a pulverized coal burner suitable for lowering the concentration of nitrogen oxides (hereunder, referred to as NOx).
In general, for burners, suppression of NOx formation during combustion is a subject matter to be solved. Particularly, coal includes a larger amount of nitrogen, compared with gaseous fuel and liquid fuel. Therefore, it is more important to decrease NOx produced by combustion of pulverized coals than in a case of combustion of gaseous fuel or liquid fuel.
NOx produced by combustion of pulverized coals is almost all NOx that is produced by oxidizing nitrogen contained in coal, that is, so-called fuel NOx. In order to decrease the fuel NOx, various burner structures and burning methods have been studied.
As one of the burning methods, there is a method forming a low oxygen concentration region within a flame and reducing (deoxidizing) NOx. For example, JP A 1-305206 (U.S. Pat. No. 4,930,430), JP A 3-211304, JP A 3-110308, U.S. Pat. No. 5,231,937, U.S. Pat. No. 5,680,823, etc. disclose a method of producing a flame of low oxygen concentration atmosphere and completely burning coal, and a structure having a fuel nozzle for pneumatically transferring coal at the center thereof and an air injecting nozzle arranged outside the fuel nozzle. According to this prior art, a reducing flame region of a low oxygen concentration is formed within the flame, reducing reactions of NOx are progressed in the reducing flame region, and an amount of NOx occurred within flame is suppressed to be small. Further, JP A 1-305206 discloses a method of stabilization of flame by providing, at an outlet end portion of a nozzle, an obstacle against the flow direction of gas. Further, JP A 3-311304, JP A 3-110308 and U.S. Pat. No. 5,231,937 disclose stabilization of flame by providing a flame stabilizing ring at the tip of a pulverized coal nozzle. According to this prior art, recirculating zones are formed downstream of the tip of the pulverized coal nozzle by providing the flame stabilizing ring or obstacle at the tip of the pulverized coal nozzle. Since a high temperature gas stays in the recirculating zones, ignition of pulverized coals progresses and the stability of the flame can be raised.
However, in the above-mentioned prior art, NOx formation has not been sufficiently suppressed as yet.